How to popup simple message box from python console?
One way to do this is by using a operator and the self.report() method. Here is a example 

import bpy

class MessageBoxOperator(bpy.types.Operator):
    bl_idname = "ui.show_message_box"
    bl_label = "Minimal Operator"

    def execute(self, context):
        #this is where I send the message
        self.report({'INFO'}, "This is a test")
        return {'FINISHED'}

bpy.utils.register_class(MessageBoxOperator)

# test call to the 
bpy.ops.ui.show_message_box()


This will show a little message box up in the top tool bar and really isn't efficient for a bigger/longer message.

Another more efficient way of displaying a message box is window_manager.popup_menu() here is an example script:

import bpy

def ShowMessageBox(message = "", title = "Message Box", icon = 'INFO'):

    def draw(self, context):
        self.layout.label(message)

    bpy.context.window_manager.popup_menu(draw, title = title, icon = icon)


#Shows a message box with a specific message 
ShowMessageBox("This is a message") 

#Shows a message box with a message and custom title
ShowMessageBox("This is a message", "This is a custom title")

#Shows a message box with a message, custom title, and a specific icon
ShowMessageBox("This is a message", "This is a custom title", 'ERROR')


You can see hear I made a custom message box method called ShowMessageBox() this is what the method will show starting from the first example to the third: 







Hope this helps! (sorry for the super late reply)

EDIT for 2.80

UILayout.label(text="") requires text to be a keyword argument.

self.layout.label(text=message)